Terms used in this specification are defined prior to descriptions of conventional techniques.
An “optical path deflecting element” refers to an optical element that is capable of switching an optical path of light by deflecting the optical path, that is, by parallel shifting exit light with respect to incident light or rotating it by a certain angle, in response to an external electric signal, or by combining both of them. In the following descriptions, the degree of parallel shifting with respect to optical path deflection through the shifting is referred to as a “shift amount” and the degree of rotation with respect to optical path deflection through the rotation is referred to as a “rotation angle”. Also, an “optical deflecting device” refers to a device which deflects the optical path of light and includes the optical deflecting element as described above.
A “pixel shift element (picture element shifting element)” refers to an optical path deflecting device of an image displaying apparatus which includes, at least, an image displaying element in which plural picture elements capable of controlling light according to image information are two-dimensionally arrayed, a light source which illuminates the image displaying element, an optical member for observing an image pattern displayed on the image displaying element, and the optical path deflecting device which deflects an optical path between the image displaying element and the optical member, for each of plural sub-fields provided by temporally dividing an image field, in which apparatus the number of picture elements of the image displaying element is apparently increased by displaying an image pattern whose display is positionally shifted in accordance with the deflection of an optical path for each sub-field which is provided by the optical path deflecting element, thereby conducting display. Therefore, basically, the optical path deflecting element or optical path deflecting device defined above may be used as the optical deflecting means (a pixel shift element (picture element shifting element)).
Conventionally, various kinds of techniques relating to, for example, an optical path deflecting element (or optical deflection element) or pixel shift element using liquid crystal material and an image displaying apparatuses using it have been proposed (e.g., see Japanese Patent Application Publication No. 06-018940, Japanese Patent Application Publication No. 09-133904, Japanese Patent No. 2939826, Japanese Patent Application Publication No. 05-313116, Japanese Patent Application Publication No. 06-324320, and Japanese Patent Application Publication No. 10-133135). However, in the conventional optical path deflecting elements or pixel shift elements have various problems including the following:                high cost, large size of device, light loss, generation of optical noise such as ghosts, and/or image resolution decrease, due to complexity of configuration;        problems relating to positioning accuracy, durability, oscillation, and sound, particularly in applications where a movable component is used; and        problems relating to response speed in nematic liquid crystals, etc.        
In such a situation, the inventors or applicant have/has previously proposed an optical path deflecting element with a particular configuration (see Japanese Patent Application Publication No. 2002-328402) for the purposes of providing an optical path deflecting element or device which may mitigate the problems in the conventional optical path deflecting elements, that is, problems such as high cost, large size of device, light loss, and generation of optical noise, due to complexity of configuration, have a simple and compact configuration and low light loss, low optical noise and low image resolution decrease, and reduce the cost thereof.
The optical path deflecting element 1 includes a pair of transparent substrates 2 and 3, an orientation film 4 provided on at least one of the substrates 2 and 3, a liquid crystal 5 which fills in between the substrates 2 and 3 and which is in a chiral smectic C phase for providing a homeotropic orientation, and at least one pair 6 of electrodes 6a and 6b for applying electrical field to the liquid crystal 5, wherein the electrode pair 6 is connected to a power source 7 so that the electrical field may be applied to a layer of the liquid crystal 5. Since the optical path deflecting element utilizes the liquid crystal 5 in a chiral smectic C phase, problems such as high cost, large size of device, light loss and optical noise, due to complexity of configuration, may be mitigated, and the lower responsiveness in, for example, the conventional smectic A-type liquid crystals or nematic liquid crystals may be improved compared to the conventional optical path deflecting elements so that high speed response can be attained.
However, in order to obtain a practical optical path shift amount of several μm to several dozen μm with such an optical path deflecting element, it may be necessary to set the thickness of the liquid crystal layer to a very large thickness of several dozen μm to several hundred μm (e.g., see “Crystal Optics” Japan Society of Applied Physics, Optical Society of Japan, page 198). Generally, when the thickness of a liquid crystal layer is increased, the influence of orientation regulation power from the surface of a substrate may be reduced at the center portion of the liquid crystal layer, and thereby, it may be difficult to maintain uniform orientation throughout the liquid crystal layer. For example, the degree of orientation at the center portion of the liquid crystal layer may be lowered so as to cause white turbidity. Therefore, it may be the most important issue to attain and maintain uniform orientation throughout the liquid crystal layer in an optical path deflecting element as described above.
For this reason, the inventors or applicant have/has previously proposed, for example, an optical path deflecting element including a liquid crystal layer made of a liquid crystal material which does not form a smectic A phase at a temperature higher than that of a chiral smectic C phase (see Japanese Patent Application Publication No. 2003-280041 and Japanese Patent Application Publication No. 2002-328402) and a method including the steps of containing, for example, the monomer(s) of a polymeric material in a liquid crystal layer, maintaining the temperature of the liquid crystal layer to that for providing the smectic A phase so as to adjust the orientation of a molecule, conducting photo-polymerization to form a fibrous or network system of the polymeric material, and then cooling it to a temperature for providing the chiral smectic C phase (see Japanese Patent Application Publication No. 2004-184522). However, the freedom of the selection of liquid crystal materials may be limited with respect to the technique disclosed in Japanese Patent Application No. 2003-280041, and a side effect such as an influence on response speed or optical characteristics due to the existence of a polymeric system may be caused with respect to the technique disclosed in Japanese Patent Application No. 2004-184522.
Also, there has been proposed a method of adjusting, for example, the quantities, kinds and relative concentrations of added chiral compounds, and the spiral pitch of a nematic (N*) phase of a ferroelectric liquid crystal mixture having a large spontaneous polarization and excellent orientation for such a mixture and a displaying element using it (see Japanese Patent No. 3034024). The disclosure in this document is directed to a displaying element that uses surface-stabilized ferroelectric liquid crystal with a planar orientation.
It is disclosed that, in surface-stabilized ferroelectric liquid crystal elements, generally, it is necessary to set the thickness of a liquid crystal layer to approximately 2 μm in order to obtain the uniform orientation (single planar orientation: disintegrated-spiral monodomain orientation), high-speed response, and good contrast, and particularly, the spiral pitch with respect to the nematic (N*) phase has to be approximately five or greater times the thickness of the liquid crystal layer, namely, approximately 10 μm or greater, in order to realize single planar orientation. Also, there is disclosed an example of a method for adding a dopant for satisfying such conditions. Furthermore, it is disclosed that the spiral pitch has to be increased for a displaying apparatus with a layer which is thicker than usual, for example, a displaying apparatus which operates in guest-host mode, and it is shown that the increase of spiral pitch due to the mixture disclosed in the document may be effective. Also, since the spiral pitch is sufficiently large relative to the thickness in an operating temperature range of a surface-stabilized element, the influence of the spiral pitch with respect to the Smectic C phase on operations of the element may be negligible.
On the other hand, the design concept of a liquid crystal material for surface-stabilized one as described above may not be applied for a liquid crystal element in which a chiral smectic C phase lies in a vertical orientation (spiral retaining orientation) and the thickness of a liquid crystal layer is very large, namely, approximately several dozen μm, such as a liquid crystal element used in an optical path deflecting element according to the present invention.
That is, only the improvement of orientation or increase of spontaneous polarization by means of adjustment of the spiral pitch with respect to a nematic (N*) phase, as disclosed in Japanese Patent No. 3034024, is insufficient, and therefore, it may be important to improve the characteristic of such an element by optimizing the spiral pitch with respect to a chiral nematic C phase within an operating temperature range. For example, when a liquid crystal mixture in which the spiral pitch with respect to a nematic (N*) phase is sufficiently large is used, the spiral pitch with respect to the smectic C phase tends to increase. If such a material is used to construct a vertical orientation liquid crystal element with a liquid crystal layer having a very large thickness of approximately several dozen μm, uniform orientation is not necessarily attained and a certain liquid crystal domain may be easily generated during operation, so that characteristic degradation may be caused by means of light scattering at domain walls.